<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>喵</title>
    <style>button{
        width: 100px;
        height: 100px;
        border-radius: 50px;
    }
    </style>
    <script>var btn=document.getElementsByClassName("btn")[0];
   btn 
   </script>
</head>
<body>
   <button class="bnt">按钮</button>
   <button >按钮</button>
</body>
</html>